This invention relates to a knock suppression apparatus for an internal combustion engine, such as an automotive engine. More particularly, it relates to a knock suppression apparatus which can prevent fluctuation of the ignition timing of an engine when the engine is operating near the border between a light load and a heavy load region.
Engine knocking is a form of abnormal engine vibration which occurs when the ignition timing of an internal combustion engine is improper. The occurrence of knocking in an engine not only decreases the ride comfort and worsens the fuel economy of the vehicle in which the engine is installed, but it can also damage the engine. Therefore, automotive engines are typically equipped with knock suppression devices which detect the occurrence of knocking and then adjust the ignition timing in the lag angle direction until the knocking stops.
In one form of conventional knock suppression apparatus for an engine, a basic ignition timing for the engine is calculated based on engine operating conditions such as the engine rotational speed. In the absence of knocking, ignition is carried out using this basic ignition timing. When knocking is sensed, the basic ignition timing is corrected in the lag angle direction by a correction factor to obtain a corrected ignition timing, and ignition is carried out using the corrected ignition timing.
Various vibrations due to causes other than knocking are produced during the operation of an engine, and there is the possibility of these vibrations being mistakenly sensed as knocking vibrations, especially when the engine is operating in a light load region. Therefore, in order to prevent the ignition timing from being unnecessarily retarded due to mistaken sensing of knocking, in some conventional knocking suppression devices, the correction factor is set to zero when it is sensed that the engine has entered a light load region so that the engine will operate with the basic ignition timing, since there is a low probability of knocking occurring in this region.
However, if the engine load is fluctuating between a light load region and a heavy load region, the correction factor will be caused to abruptly switch between a zero and nonzero value, and hunting will take place, which is undesirable.
Furthermore, if it is erroneously sensed that the engine has entered a light load region due to signal noise, for example, immediately setting the correction factor to zero will cause the ignition timing to immediately revert to the basic ignition timing. Therefore, if knocking was taking place at the time that the correction factor was set to zero, the knocking will not be suppressed, and engine damage may occur.